The semiconductor industry's continuing drive toward advanced integrated circuits with increased performance and higher device packing densities has required an accompanying decrease in the minimum feature size of devices. Therefore, each new generation of advanced integrated circuit has required an increase in lithographic resolution. Traditionally, optical lithography has been used to define integrated circuit patterns on semiconductor substrates. However, current optical lithography systems have great difficulty defining photoresist patterns with feature sizes of less than 0.35 microns, which are required for next generation devices.
X-ray lithography has been proposed for fabricating next generation integrated circuits because it has a relatively large process latitude, as compared to optical lithography. The ultra-short wavelengths of the x-rays eliminate the diffraction effects that limit the resolution of optical lithography systems, and thus higher resolution can be achieved with x-ray lithography. X-ray lithography also affords a greater depth of focus over optical lithography. In addition, x-rays are relatively penetrating and therefore particles on a x-ray mask or on a photoresist layer are transparent and thus they do not contribute to defect formation during the integrated circuit fabrication process.
Existing x-ray lithography systems, however, use lithographic exposure spectrums with photon energy distributions ranging from about 800 electron volts (eV) to about 2000 eV, and this exposure spectrum adversely effects the formation of photoresist circuit patterns having small feature sizes. Therefore, the formation of advanced integrated circuits with x-ray lithography is limited. Accordingly, a need exists for an improved x-ray lithography process.